In the fields of organic chemistry, biochemistry, medicine, and the like, liquid chromatography has been widely used for measurement or analysis of a component in a sample. In the medical field, for example, liquid chromatography has been employed for measuring hemoglobin A1c, which is an indicator for diabetes diagnosis. Hemoglobin A1c is glycosylated hemoglobin that has blood sugar chemically bound to an N-terminus of a β chain of hemoglobin. A proportion of hemoglobin A1C in hemoglobins, that is, a proportion of glycosylated hemoglobin in a sum of glycosylated hemoglobin and non-glycosylated hemoglobin is considered to reflect an average blood sugar level in a period of one to two months. Therefore, a hemoglobin A1c value (%) which represents the proportion of hemoglobin A1c in hemoglobins has been widely used as an indicator for diabetes diagnosis because the value does not show temporary fluctuation unlike a blood sugar level.
Filters such as an in-line filter for filtering foreign substances are located in a flow channel between a liquid chromatography sample-injection device to a column. Such filters are disposed in order to prevent foreign substances from clogging the channel, particularly the column body, and thereby changing the pressure of a supplied liquid. Particularly a prefilter, which is located on the upstream side of the column to filter foreign substrates, is an important filter linking directly to the column clogging caused by the foreign substances.
Examples of the foreign substances, which are captured on the filters, include: those contained in a mobile phase, a reaction reagent, and the like; those from a part of an analyzer, such as a feed pump; and those derived from a sample. These foreign substances are adsorbed on the surface of filler particles filled in the column or to a detector cell, and as a result, may adversely affect the measurement or analysis. Accordingly, there have been developed various filters for efficiently capturing these foreign substances.
However, when the filtration efficiency of the filters is increased in order to more efficiently capture the foreign substances, the foreign substances may more easily clog the filters to cause the change in the supplied liquid pressure. Particularly when a large number of samples are continuously measured as in analysis of hemoglobins or when a sample high in foreign substances like a hemolyzed blood sample is measured, the foreign substances clog the filter and often cause an increase in the supplied liquid pressure. The change in the supplied liquid pressure may lead to a failure in accurate and quick measurement or analysis of the sample.
Examples of a common method of suppressing the increase in the supplied liquid pressure, caused by clogging of the filter, include a method of increasing a filtration area of the filter, a method of increasing a porosity of the filter, and a method of modifying a configuration of the filter. However, when the filtration area or the porosity of the filter is increased too much, the sample or the mobile phase in the filter may be diffused too much, which reduces the accuracy of the measurement or analysis.
As an example of modifying a configuration of the filter, Patent Document 1 discloses a method of using a two-layer filter composed of layers with different pore sizes. Patent Document 2 discloses a method of using a two-layer structure composed of a filter paper sheet and a filter. These methods are designed to prevent the clogging attributed to the foreign substances by a combination use of the two different filters without decreasing the filtration efficiency for the foreign substances. However, these ideas have been established by taking only the filter into consideration, and no studies have been made covering the column where the clogging attributed to the foreign substances poses a problem as in the filter.
For example, a column for analysis of hemoglobins, which is used for measuring a hemoglobin A1c value, is usually replaced each time after the measurement of 1500 to 3000 samples is completed, although depending on the number of samples guaranteed by a manufacturer. In contrast to this, the prefilter, which is located on the upstream side of the column, is usually replaced each time after the measurement of hundreds of samples is completed. Thus, the filters require the replacement more often than the column does, which is a significant burden in terms of operation and cost.    Patent Document 1: Japanese Kokai Publication No. Hei-02-262054 (JP-A Hei-02-262054)    Patent Document 2: Japanese Kokai Publication No. Hei-05-203634 (JP-A Hei-05-203634)